Asymmetrical thyristor with blocking/sweep voltage independent of temperature behavior

ABSTRACT

Both the blocking voltage as well as the sweep voltage of conventional thyristors exhibit a pronounced temperature behavior, whereby the corresponding voltage values can change by up to 15% within the relevant temperature range (5° C.-120° C.). 
     In the proposed thyristor, the overhead triggering is compelled by the “punch through” effect that is independent of the temperature (expanse of the space charge zone allocated to the p-base/n-base junction  10 ) up to the neighboring n-base/p-emitter junction  11 ). Due to the laterally non-uniform distribution of the dopant in the n +  stop zone ( 7 ′) of the anode-side base ( 7 ), further, it is assured that the central thyristor region always ignites first. 
     Sweep or punch through voltage is not dependent on the temperature in the asymmetrical thyristors.

BACKGROUND OF THE INVENTION

Both the blocking voltage as well as the sweep voltage (blocking voltageafter which the thyristor switches into the conductive condition) of athyristor exhibit a pronounced temperature behavior. Thus, the blockingand the sweep voltage initially continuously increase with thetemperature, reach a maximum, in order to ultimately drop tocomparatively low values. Whereas the influence of the positivetemperature coefficient of the avalanche coefficients characterizing theelectron multiplication by impact ionization predominates at low andmoderate temperatures, the drop of the blocking and sweep voltage athigher temperatures T≧100° C. can be attributed to the dominance of thepositive temperature coefficient of the transistor current gain α_(pnp)as a result of the greatly increasing blocking current. The temperaturedependency of the blocking and sweep voltage has an especiallydisturbing influence in highly inhibiting thyristors that exhibit aprotection against overhead ignition integrated in the semiconductorbody. Given these thyristors, the blocking and the sweep voltage canchange by up to 15% in the relevant temperature range (5° C.-120° C.).For example, the sweep voltage thus rises from U_(BO)=8.0 kV to valuesU_(BO)≈9.2 kV when the thyristor heats from T=23° C. to T=90° C. duringoperation.

The user must take this effect into account with a more complicatedwiring of the thyristor. The manufacturer of the component, by contrast,is compelled to keep the scatter of the parameters (basic doping of theSi substrate, dopant profiles, contour of the edge termination, etc.)that influence the blocking or respectively, sweep voltage extremelylow. The product becomes substantially more expensive due to the hightechnological outlay given simultaneously reduced yield that accompaniesthis.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an asymmetricalthyristor whose sweep voltage is not dependent on the temperature oronly insignificantly dependent on the temperature. This is achieved by alateral non-uniform distribution of the dopant in the stop zone of theanode-side base. A stop zone having a comparatively lightly dopedcentral region and a more highly doped outside region can be simply andcost-beneficially manufactured (“masked” implementation).

In general terms the present invention is an asymmetrical thyristorhaving a semiconductor body provided with a first electrode serving ascathode and with a second electrode serving as anode, whereby thesemiconductor body has a plurality of differently doped regions. Thedoping and position of the regions are prescribed such that the regionsform a cathode-side emitter of a first conductivity type, a cathode-sidebase of a second conductivity type, an anode-side base of the firstconductivity type and an anode-side emitter of the second conductivitytype. A stop zone of the first conductivity type is located in theanode-side base. The stop zone has a central region lying under atrigger contact or under a light-sensitive structure and a regionlaterally adjoining thereto. The stop zone is more lightly doped in thecentral region than in the laterally adjoining region. The anode-sideemitter is provided with anode shorts.

Advantageous developments of the present invention are as follows.

The doping of the central region and of the laterally adjoining regionof the stop zone are selected such that the space charge zone of thep-junction separating the cathode-side and the anode-side base reachesthe anode-side pn-junction only in the central region of thesemiconductor body given a predetermined value of the difference betweencathode and anode potential.

The metallization arranged on a cathode-side principal surface of thesemiconductor body contacts both the cathode-side base as well as anauxiliary emitter of the first conductivity type embedded into thecathode-side base.

The laterally non-uniform distribution of the dopant concentration inthe stop zone is effected in that a dopant in the central region of thestop zone is implanted in a dose lower by a factor of 100-5000 than inthe laterally adjoining region. In one embodiment the dose of the dopantin the central region of the stop zone is in a range of 0.3-2·10¹² cm⁻².

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel,are set forth with particularity in the appended claims. The invention,together with further objects and advantages, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings, in the several Figures of which like referencenumerals identify like elements, and in which:

FIG. 1 shows an exemplary embodiment of the inventive thyristor incross-section; and

FIG. 2 shows the dopant profile of the asymmetrical thyristor under thecathode metallization.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The thyristor shown in cross-section in FIG. 1 has a dynamicallybalanced structure with respect to the axis 4 residing perpendicular onthe two principal surfaces 2/3 of the semiconductor body 1. Thewafer-shaped semiconductor body 1 composed of silicon comprises aplurality of differently doped regions 5-8 respectively separated bypn-junctions 9-11 having the dopant profiles shown in FIG. 2. Theseregions of different conductivity form the n⁺ emitter 5 contacted by thecathode 12, the cathode-side p-base 6, the only lightlyelectron-conducting n-base 7 provided with a n⁺ stop zone 7′ (bufferlayer), as well as the p⁺ emitter 8 of the thyristor contacted by theanode 13. In order to suppress the influence of the leakage current onthe transistor gain factor α_(pnp) given low leakage current densities,the p⁺ emitter 8 is advantageously equipped with anode shorts 19. Thestructure 15/16 arranged between the gate electrode 14 and the cathode12 is usually referred to as amplifying gate. It is composed of an n⁺doped region 15 (auxiliary emitter) embedded into the cathode-side base6 and of a metallization 16 that contacts both the n⁺ doped regions 15as well as the p base 6. In collaboration with the semiconductor layerslying therebelow and with the anode 13, this structure 15/16 forms anauxiliary or pilot thyristor serving as driver stage for the mainthyristor that considerably intensifies the controlled current suppliedinto the p base 6 via the gate electrode 14. The emitter shorts 17present in the region of the cathode 12 assure that the thyristor doesnot already ignite an uncontrolled fashion before the static sweepvoltage is reached given a high dU/dt load of a few 1000 V/μs.

Differing from the prior art, the overhead ignition of the inventivethyristor is not compelled by an avalanche but by what is referred to as“punch-through” effect. What is to be understood as “punch through” inthis case is the expanse of the space charge zone of the p-base/n-basejunction 10 polarized in non-conducting direction up to the neighboringn-base/p-emitter junction 11 polarized in conducting direction and thesteep rise of the leakage current resulting therefrom within thesemiconductor structure comprising the two pn-junctions 10/11. Sincethis effect producing the punch-through is essentially dependent only onthe basic doping and on the thickness of the n-base 7 and is no longerdependent on the temperature, the sweep voltage also no longer changesor changes only insignificantly in the range of the operatingtemperatures (5° C.-120° C.). In order to assure that the ignitiontriggered by “punch-through” begins in the center of the thyristor, thedopant concentration of the n⁺ stop zone 7′ in the area 18 (radius R≈1-3mm) lying under the gate electrode 14 is clearly reduced compared to thelaterally adjoining region of the stop zone 7′ that extends up to thewafer edge. The comparatively light doping in the central region 18 ofthe stop zone 7′ means that the space charge zone allocated thereat tothe pn-junction 10 expands given application of a high sweep voltage upto the p⁺ emitter 8, the blocking current consequently increasesgreatly, and the central thyristor region switches into the conductivecondition. Due to the higher doping of the n⁺ stop 7′, by contrast, thespace charge zone in the laterally adjoining regions of the componentcannot reach the n-base/p-emitter junction 11, this preventing a currentrise caused by punch through.

The laterally non-uniform distribution of the dopant in the n⁺ stop zone7′ can be produced, for example, by a two-stage implantation with donoratoms (usually phosphorous atoms) undertaken at the anode side. Thecorresponding method is characterized by the following steps:

surface-wide, anode-side implantation of phosphorous ions with a doseof, for example, 0.3 through 2·10¹² cm⁻², which enables punch through;

application of a photoresist or SiO₂ layer serving as implantation maskand having the thickness d=1 μm in the central region of thesemiconductor body 1;

anode-side, masked implantation of phosphorous ions with a dose or, forexample, 3-10·10¹⁴ cm⁻², which prevents punch through;

stripping the photoresist or SiO₂ layer; and

drive-in of the phosphorous atoms at high temperature (T≈1240° C.)within a time span of approximately 10 through 30 hours.

With a given dopant concentration in the p⁺ emitter 8, the transistorgain α_(pnp) and, thus, the height of the blocking voltage as well asthe pass voltage of the thyristor can be influenced by the doping of then⁺ stop zone 7′ outside the central region 18 (PESC 88 Record, April1998, Pages 934-935). It must thereby be taken into consideration,however, that the phosphorous implantation dose that determines thedopant concentration can be varied only within a relatively narrow rangein order to assure the required, high blocking voltage (α_(pnp)optimally low) given simultaneously low pass voltage (α_(pnp) optimallyhigh).

The above comments are directed to a gate-controlled thyristor. Ofcourse, the temperature dependency of the sweep voltage of alight-triggerable, asymmetrical thyristor can also be clearly reduced inthat the region of the stop zone lying under the radiation-sensitivethyristor structure is more likely doped than the laterally adjoiningregion.

The invention is not limited to the particular details of the apparatusdepicted and other modifications and applications are contemplated.Certain other changes may be made in the above described apparatuswithout departing from the true spirit and scope of the invention hereininvolved. It is intended, therefore, that the subject matter in theabove depiction shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:
 1. An asymmetrical thyristor, comprising: asemiconductor body having a first electrode serving as cathode and asecond electrode serving as anode, and the semiconductor body having aplurality of differently doped regions; the differently doped regionsbeing positioned such that the regions form a cathode-side emitter of afirst conductivity type, a cathode-side base of a second conductivitytype, an anode-side base of the first conductivity type and ananode-side emitter of the second conductivity type; a stop zone of thefirst conductivity type located in the anode-side base; the stop zonehaving a central region, lying under one of a trigger contact or alight-sensitive structure, and a region laterally adjoining thereto; thestop zone being more lightly doped in the central region than in thelaterally adjoining region; and the anode-side emitter being providedwith anode shorts.
 2. The asymmetrical transistor according to claim 1,wherein the doping of the central region and of the laterally adjoiningregion of the stop zone are selected such that a space charge zone of ap-junction separating the cathode-side base and the anode-side basereaches an anode-side pn-junction only in the central region of thesemiconductor body given a predetermined value of a difference betweencathode potential and anode potential.
 3. The asymmetrical thyristoraccording to claim 1, wherein metallization arranged on a cathode-sideprincipal surface of the semiconductor body contacts both thecathode-side base and an auxiliary emitter of the first conductivitytype embedded into the cathode-side base.
 4. The asymmetrical thyristoraccording to claim 1, wherein a laterally non-uniform distribution ofdopant concentration in the stop zone is effected in that a dopant inthe central region of the stop zone is implanted in a dose lower by afactor 100-5000 than in the laterally adjoining region.
 5. Theasymmetrical thyristor according to claim 4, wherein the dose of thedopant in the central region of the stop zone is in a range of0.3-2·10¹² cm⁻².